1. Field of the Invention
The present invention relates to a solid electrolytic capacitor and a method producing the same.
2. Description of the Related Art
In the past, since solid electrolytic capacitors using tantalum, aluminum, or the like have a large electrostatic capacity and an excellent frequency characteristic, they have been widely used for electronic devices such as portable electronic terminals and personal computers. In late years, with higher reliability and higher performance of the electronic device, requirements of leakage current (LC) reduction, lower equivalent series resistance (low ESR) and the like are further increased.
Here, a configuration of a conventional solid electrolytic capacitor is explained. FIG. 3 is a schematic cross-sectional view for explaining the configuration of the conventional solid electrolytic capacitor. Anode body 21 is a sintered body having a lot of fine pores (porous layer) obtained by forming and sintering fine particles of a valve action metal such as tantalum or aluminum. Anode lead 28 which comes to be an anode part, as well as anode body 21, includes a wire or the like of a valve action metal, and is led from the anode lead leading surface (leading surface) of anode body 21. Dielectric layer 22 of an oxide film is formed on the surface of anode body 21 and on the surface inside the polar layer. Further, solid electrolyte layer 23 is formed on the surface of dielectric layer 22. Insulation part 30 includes an epoxy resin and is placed at the base part of anode lead 28 so as to prevent an electrical short circuit by connecting solid electrolyte layer 23 to anode lead 28.
Graphite layer 24 and silver paste layer 25 as a cathode part are formed on the surface of solid electrolyte layer 23 to configure a capacitor element. Anode lead 28 and the cathode part of the capacitor element are electrically connected to lead frame 31 and lead frame 32 that are external electrode terminals by welding or with electroconductive adhesive 26. Then, an outer package including outer resin 29 is formed to obtain a solid electrolytic capacitor.
The solid electrolyte layer has a function to electrically connect the dielectric layer and the cathode part and to extract an electrostatic capacity of the dielectric layer. The solid electrolyte layer is one of the important constituents for obtaining electrical characteristic of the solid electrolytic capacitor, and the configuration, the production method and the like are studied. In the late years, a solid electrolytic capacitor using an electroconductive polymer layer including an electroconductive polymer as the solid electrolyte layer is produced.
For forming the electroconductive polymer layer, chemical oxidative polymerization method is conventionally used, in which the anode body (anode body element) where the dielectric layer is formed is immersed in a solution obtained by adding a monomer, a catalyst, oxidant that is a dopant and the like to a solvent, and in which it is polymerized on the surface of the dielectric layer. In chemical oxidative polymerization method, a solvent with a low viscosity which has a good permeability and easily permeates the detail of the porous layer is generally used to obtain an electroconductive polymer layer having a characteristic that the adhesion between the dielectric layer and a layer formed as the cathode part is made good, etc.
Also, as well as chemical oxidative polymerization method, a method with an electroconductive polymer suspension is used, too. The method with an electroconductive polymer suspension (electroconductive polymer suspension method) is a method in which the anode body element is immersed and impregnated to an electroconductive polymer suspension containing an electroconductive polymer which is preliminary polymerized and to which a dopant is added, and it is pulled up, and it is then dried by heating to form an electroconductive polymer layer. The electroconductive polymer layer obtained by this method has characteristics that the density is high and the heat resistance is good in comparison with the electroconductive polymer layer obtained by chemical oxidative polymerization method or the like, and that the electroconductive polymer layer can be quickly formed. The electroconductive polymer suspension used in this method and the production method is disclosed in claims 2 and 3, and paragraphs 0014 to 0028 of JP 11-121281 A.
As a condition of the electroconductive polymer layer formed on the dielectric layer by electroconductive polymer suspension method, as shown in FIG. 3 described above, there is a case in which the electroconductive polymer layer (solid electrolyte layer 23) also covers the surface of the leading surface. The example of the solid electrolytic capacitor having this configuration is disclosed in FIG. 1 and paragraph 0016 of JP 2010-3772 A.
As described above, although the electroconductive polymer layer obtained by chemical oxidative polymerization method easily permeates the detail of the porous layer because a solution with a low viscosity is used, the electroconductive polymer layer formed has a low density due to the solution character. Therefore, there is a concern that the electroconductive polymer layer is compressed by the mold forming of the outer resin, and that the dielectric layer is connected to the graphite layer, these result in occurring an electrical short circuit or increasing LC. Also, there is a concern that a gas (oxygen) easily permeates it because of the low density, and that the increase of ESR occurs by the oxidation inside the anode body.
Against these concerns, it is effective to form an electroconductive polymer layer by electroconductive polymer suspension method by which an electroconductive polymer layer with a high density is obtained. In other words, if an electroconductive polymer layer with a desired thickness can uniformly be formed on the surface of the anode body element, high reliability against the oxygen permeation and the pressure in mold forming can be realized. Therefore, a solid electrolytic capacitor obtained by forming an electroconductive polymer layer by using an electroconductive polymer suspension on the surface of the electroconductive polymer layer by chemical oxidative polymerization method is studied.
In the case where the electroconductive polymer layer with a high density is formed by using electroconductive polymer suspension method on the whole surface of the anode body element having the electroconductive polymer layer with a low density, a residual air or solvent inside the pore of the anode body element may be trapped. If the heating such as drying is carried out in this situation, there is a problem that the air or solvent may be expand to generate small delamination at the interface of the electroconductive polymer layer and the dielectric layer, and that the product yield is lowered by the increase of ESR or the like.
Thus, the object of the present invention is, by solving the above-mentioned problem, to provide a solid electrolytic capacitor and a method for producing the same, in which the increase of ESR is suppressed, in which high reliability is realized, and further in which the electrical short circuit or the like is suppressed and in which the product yield is improved.